Composite materials are widely used in the fabrication of products ranging from tennis rackets to advanced aerospace structure. Composite materials are sold in various forms including sheets consisting of continuous parallel reinforcing fibers embedded in an organic matrix material. Typically, the matrix material is an adhesive, such as an epoxy resin. The sheet consisting of the reinforcing fibers and the matrix material is known as "prepreg."
Prepreg is commonly provided by a manufacturer as a continuous sheet wound on a roll or spool, with the fibers extending longitudinally around the spool. Prepreg can be purchased in widths ranging from about 1/8 inch wide to several meters wide. Typically, wider prepreg spools are used in hand lay-up procedures, in which individual layers or plies of prepreg are manually placed on top of each other by a worker. Narrower spools, commonly referred to as tapes or tows, are typically used in automated tape laying machines, fiber placement machines or winding machines. Automated tape laying machines use wider spools of prepreg on the order of inches while fiber placement machines use narrower spools on the order of 1/8 to 1/2 inch. Throughout this application, the terms automated tape laying machine and fiber placement machine are used interchangeably.
Fiber placement machines use a robotic arm to mechanically place individual layers or plies of composite prepreg onto an underlying substrate. Fiber placement machines are capable of starting an individual ply of prepreg at the location desired, laying up the ply over the distance desired, and then terminating or dropping off the ply at the location desired. As shown in FIG. 1, the operative end of the robotic arm terminates at a fiber placement head 10. One or more continuous spools of prepreg tape or tow are mounted on the fiber placement head. Most fiber placement machines can place a maximum of 12 to 32 tows at one time, forming a band of prepreg from 1.5 to 4.0 inches wide. The individual tows 12 are aligned and organized into a prepreg band by a band collimator 14. The collimated composite band extends through a series of rollers 16 and a tow cutter and clamp mechanism 18, as shown schematically in FIG. 1. The tow cutting and clamp mechanism 18 is capable of cutting or restarting the entire band or individual tows forming the band during the composite lay-up operation. Alter being collimated, the band of composite material passes between a compaction roller, rotatably coupled to the fiber placement head 10, and the underlying substrate 26.
As the fiber placement head 10 is moved over the substrate 26, as illustrated by arrow 28, the compaction roller 22 presses the prepreg band against the substrate. The pressure causes the prepreg band to adhere to the substrate 26. As the fiber placement head 10 moves forward, the portion of the prepreg band rearward of the compaction roller is maintained in position by the adhesive adherence between the band of prepreg and the substrate.
A fiber placement machine's use of multiple tows permits the fiber placement head to be steered along the tool surface while allowing each tow to travel at its own speed. This differential tow pay-out capability coupled with the ability to individually drop or add tows provides flexibility required by rapidly changing substrate contours and sizes.
The use of multiple tows provides increased flexibility, however it also introduces a potential for creating gaps between individual tows during placement. The fiber placement machine's ability to accurately place individual composite tows is influenced by a number of different variables including prepreg tack, the contours of the substrate, the speed at which the fiber placement head advances, the force provided by the compaction roller, the type of material, environmental conditions such as temperature and humidity, etc. These variables, along with others, often result in small gaps between the individual tows after they have been placed on the substrate.
Fiber placement machines apply the composite tows to the substrate at near-zero tension. The use of near-zero tension allows the fiber placement machines to place the tows on concave, convex, and compound surfaces. However, the use of near-zero tension also contributes to the occurrence of gaps between the edges of the individual tows after placement.
The size, location, and frequency of gaps between tows influences the quality of the resulting part. As illustrated in FIGS. 2 and 3, such gaps are generally of two different types. In some applications, as a composite band 30 is applied to the substrate, it is necessary to terminate or add an individual tow 32 in order to decrease or increase the width of the band placed. When an individual tow is terminated or added, a triangular gap 34 is formed at the terminal end of the tow 32. The size and shape of the gap 34 is partially determined by the width of the individual tows, the speed at which the compaction roller places the tows, and the ability of the fiber placement head to collimate the remaining tows.
In addition to gaps produced by adding or terminating individual tows, gaps are created during the placement process. As illustrated in FIG. 3, as a composite band 36 is placed on the substrate, the tows 38 forming the band do not always remain collimated. Sometimes, the individual tows move or slip during placement or after placement, forming gaps 40 between the edges of the tows.
The occurrence of gaps caused by dropping off or adding individual composite tows can be approximately calculated using the control data supplied to the fiber placement machine. In order to operate the fiber placement machine, it is necessary to model the surface upon which the composite prepreg is placed. Such modeling includes determining the location of all tow drop-offs and additions. Using such control data, the position and size of the triangular-shaped gap at the terminal end of terminated tows can be calculated. Software packages to perform such calculations are commercially available from such manufacturers as Cincinnati Milicron, Cincinnati, Ohio.
The occurrence of gaps due to factors other than tow additions or terminations are difficult, if not impossible, to accurately predict. Numerous physical phenomena contribute to the occurrence of such gaps. Some of the contributing variables include: (1) variations in tow widths, (2) twisted fibers or resin build-ups in individual tows, (3) the spool tension and surface adherence on each tow, (4) the steering radius of the fiber placement head or radius of curvature of the compaction. roller, (5) overlap, foreign materials, or gaps in the underlying substrate, (6) environmental factors such as temperature and humidity affecting the behavior of the individual tows, etc.
The occurrence of gaps between individual tows is also influenced by the contours of the substrate on which the tows are being placed. As illustrated in FIG. 4, gaps are less likely to occur when laying up planar surfaces, such as surface 42, and more likely to occur when laying up tows on surfaces having more complex geometries or acute radiuses of curvature. An estimate of the relative occurrence of gaps associated with surface contours is, from lowest to highest, planar surfaces 42, peak surfaces 44, ridge surfaces 46, saddle ridge surfaces 48, saddle valley surfaces 50, valley surfaces 52, pit surfaces 54, to minimal surfaces 56.
Another contributor to the occurrence of gaps between tows is the "traverse compliance" of the compaction roller. Traverse compliance is the potential for individual tows to lift from the surface of the substrate as the fiber placement head moves across the valley of a convex surface. Such lifting is commonly related to the diameter of the compaction roller compared to the bandwidth of the prepreg being placed.
When parts contain complex geometries or an acute radius of curvature, they are generally laid up manually. Alternatively, if possible they are laid up in a pattern such that the change in surface contour in the direction of movement of the fiber placement head is gradual enough to be accommodated by the fiber placement machine.
The high speed at which fiber placement machines place composite prepreg, along with the contours of completed pans, present a quality control challenge. Presently, there are no automated methods of monitoring the occurrence or size of gaps between individual tows after placement. Prior methods of quality control and assurance involve stopping the fiber placement head after each composite layer is placed and manually inspecting the resulting surface. Even with the assistance of hand-held inspection devices, prior methods are error-prone, time-consuming, costly, and questionable in terms of completeness and reliability. In addition, shutting down the fiber placement head during manual inspection reduces the efficiency of the fiber placement machine, thus adding to manufacturing costs.
Past inspection systems have been unsuccessful for a number of reasons. Black composite surfaces have light absorbent and low contrast properties which make imaging in ambient or fluorescent light extremely difficult. These characteristics eliminate consideration of many commercial vision systems as a solution for automated inspection of composites placed by fiber placement machines. This degree of difficulty is magnified by the fact that the surfaces and edges of the individual tows, unlike metallic surfaces, are not uniform. Fiber strands randomly crisscross the surface and extend out from the edges of the tows. These strands may appear as single spikes in a video profile of the composite surface. The fiber strands introduce a high level of image speckle noise that interferes with crisp edge definition using standard image analysis methods and techniques.
Another difficulty in producing accurate image data stems from sensor returns from a light source and camera mounted on a moving fiber placement head. Mounting fixed lights and cameras above the fiber placement machine or on the creel above the fiber placement head produces poor image results. The use of fixed mounts results in poor image resolution, partially-occluded surfaces, unreliable surface indexing, and introduces additional problems as the fiber placement head moves in and out of the camera's visual field of view.
Mounting the visual system on a fiber placement head having multiple degrees of freedom also introduces viewing difficulties. The varying angle of orientation between the fiber placement head and the surface of the substrate produces a varying angle of incidence between the visual imaging system and the surface of the substrate. It cannot be assumed that the line of sight of a visual imaging system will also be normal to the imaging surface, or that the surface will be flat. However, in a fiber placement machine, a visual image of some minimal quality can generally be obtained by mounting the visual imaging system on the fiber placement head. This is true because in most applications the most radical angle of incidence between the fiber placement head and the substrate is approximately 20.degree..
Taking image data at an instantaneous point in time requires quickly obtaining image data during operation of the fiber placement head. This is complicated by the fact that fiber placement heads can achieve high acceleration/deceleration rates. For example, a typical fiber placement head can achieve acceleration rates of approximately 40 feet/sec.sup.2 in the x, y and z directions, 100 degrees/sec.sup.2 changes in pitch and roll, and 50 degrees/sec.sup.2 changes in yaw. In addition, if the fiber placement head is being used to place composite materials on a rotating mandrel, the mandrel may be accelerating at a rate of over 30 feet/sec.sup.2. Most visual imaging systems use rapid shutter speeds or data acquisition rates that should be sufficient to overcome these speed of movement concerns. For example, many line scan cameras are capable of thousands of scans per second.
Another concern in an appropriate vision imaging system is that the registration of the image to the surface may change from one scan (frame) to the next due to the movement or vibration of the fiber placement head. This problem may be accounted for by gathering vibration data and using it in the reduction of the visual image data provided by the vision imaging system. Alternately, registration changes in the visual imaging data from frame to frame may be dampened at the pixel level.
The primary purpose of prior quality control and assurance gap detection inspections is not to improve the accuracy of the fiber placement machine. Instead, the goal of such inspections is to achieve a better measurement of the quality of the manufactured part and to quantify fiber placement repeatability within a set of statistical figures of merit.
A need exists for quality control methods and apparatus that reduce the amount of manual labor required to inspect composite materials placed by fiber placement machines. The present invention is directed to meeting this need.